1. Technical Field
The present invention relates to a protection circuit and method for an inverter apparatus which is an electric vehicle semiconductor power conversion apparatus mounted in an electric car, a hybrid car, or the like.
2. Related Art
An electric vehicle inverter apparatus is connected to a high-voltage battery unit that incorporates an inrush current suppression function and a high-voltage interrupting function and that has a voltage of hundreds of volts (V). Even when the high-voltage battery unit is disconnected therefrom, a high voltage remains in a main circuit capacitor. Countermeasures for preventing occurrence of electric shock during maintenance and in impact accidents are important from the viewpoint of safety enhancement. As one of the countermeasures, a discharge resistance or a discharge circuit has hitherto been installed in a high-voltage portion of the inverter apparatus.
FIG. 11 illustrates a discharge circuit according to Conventional Example 1 of an electric motor driving apparatus disclosed in Patent Document 1.
<Operation of Discharge Circuit Disclosed in Patent Document 1>
<<At Start-Up of Motor>>
As illustrated in FIG. 11, when a direct-current (DC) brushless motor M starts up (i.e., a power supply circuit is in an on-state), a controller 62 puts the contacts of a power supply relay Ry1 into an open state (off-state), and brings those of a charge relay Ry2 into a closed state (on-state), respectively. In addition, the controller 62 puts the contacts of a normally-closed type relay Ry3 into an open state (off-state). In this condition, electric current flowing in a discharge resistor R1 is interrupted by the contacts of the normally-closed type relay Ry3. Accordingly, electric charges are stored in a smoothing capacitor C through a charge resistor R2. That is, the smoothing capacitor C is charged. At that time, the charge resistor R2 is interposed between a power supply and the smoothing capacitor C. Thus, inrush current is prevented from flowing into the smoothing capacitor C.
<<During Operation of Motor>>
The controller 62 puts the contacts of the power supply relay Ry1 into a closed state (on-state) and brings those of the charge relay Ry2 into an open state (off-state). Consequently, the DC brushless motor M is operated by an inverter 61 of the power supply circuit at a predetermined rotational speed in a stable operation state.
<<At Stoppage of Motor>>
On the other hand, when the DC brushless motor M is stopped (i.e., the power supply circuit is in an off-state), the controller 62 puts the contacts of the power supply relay Ry1 and those of the charge relay Ry2 into an open state (off-state). In addition, the controller 62 brings the normally-closed type relay Ry3 into a closed state (on-state). In this state, electric charges stored in the smoothing capacitor C flow through the charge resistor R1 and the normally-closed type relay Ry3. Consequently, the smoothing capacitor C is discharged.
FIG. 12 illustrates a discharge circuit according to Conventional Example 2 of a power supply apparatus for a DC-DC converter or the like disclosed in Patent Document 2.
<Configuration of Discharge Circuit Disclosed in Patent Document 2>
<<Discharge Current Control Portion DL>>
As illustrated in FIG. 12, a discharge current control portion DL is constituted by a series circuit of a discharge resistor R1 and a switching element Q1. The on/off control of an electric current path between an output terminal Out and a ground point switching element Q1 is performed by turning on and off the switching element Q1. Thus, the on/off control of electric current flowing in the discharge resistor R1 can be performed.
<<Charge Storage Portion CS>>
An electric charge storage portion CS is constituted by a diode D11 serving as a rectifying element, and a capacitor C11 serving as a charge storage means. The anode of a diode D11 is connected to an output terminal Out, while the cathode of the diode D11 is connected to one of the electrodes of the capacitor C11 so that the other electrode of the capacitor C11 is connected to the ground point. The connection point between the diode D11 and the capacitor C11 is connected to a discharge control portion DC as a charge storage portion output terminal CST. With this circuit configuration, a voltage substantially the same as an output voltage can be maintained at the charge storage portion output terminal CST which is one of the terminals of the capacitor C11. Thus, this voltage is supplied to the discharge control portion DC as a control portion power supply voltage.
<<Discharge Control Portion DC>>
The discharge control portion DC is constituted by a series circuit of a transistor Q12, and resistors R16 and R17 connected between the charge storage portion output terminal CST and a ground point, and resistors R18 and R19 for performing the on/off control of the transistor Q12 by applying a bias to an input terminal (base electrode in this case) of the transistor Q12. One of the terminals of the resistor R18 is connected to the charge storage portion output terminal CST. The connection point between the resistors R18 and R19 is connected to the input terminal of the transistor Q12. The resistor R19 is connected to an input voltage detection portion ID. An input detection signal output from the input voltage detection portion ID is applied to the input terminal of the transistor Q12 as an input signal.
<<Input Voltage Detection Portion ID>>
FIG. 13 is a circuit diagram illustrating the input voltage detection portion ID shown in FIG. 12. As illustrated in FIG. 13, a series circuit of resistors R21 and R22, a series circuit of a resistor R23 and a zener diode ZD21, a power supply terminal of an operational amplifier AMP, and a series circuit of a resistor R25 and a transistor Q21 are connected between an input terminal In and a ground point. In addition, an output terminal of the operational amplifier AMP and one of the terminals of a resistor R24 connected to the input terminal In at the other terminal thereof are connected to the input terminal of the transistor Q21. The connection point between the resistors R21 and R22 is connected to the negative input terminal of the operational amplifier AMP, while the connection point between the resistor R23 and the zener diode ZD21 is connected to the positive input terminal of the operational amplifier AMP.
<<Operation 1 of Input Voltage Detection Portion ID: Output of Logical H Signal in Steady State>>
In a steady state, the voltage of the input terminal In is an input voltage itself and has a logical HIGH level. At that time, a zener diode ZD21 is tuned on. A voltage at the positive input terminal of an operational amplifier AMP is the zener voltage of the zener diode ZD21. On the other hand, a voltage obtained by voltage division with the resistors R21 and R22 is applied to the negative input terminal of the operational amplifier AMP. This voltage is set to be higher than the zener voltage. When the potential at the negative input terminal of the operational amplifier AMP is higher than that at the positive input terminal thereof, the output of the operational amplifier AMP is at a logical LOW level. Thus, the transistor Q21 is turned off. No current flows in a resistor R25 because the transistor Q21 is off. The input voltage detection portion ID outputs a signal, whose level is a logical HIGH level, to the discharge control portion DC as an input detection signal. That is, in a steady state, a signal whose level is a logical HIGH level is output from the input voltage detection portion ID.
<<Operation 2 of Input Voltage Detection Portion ID: Output of Logical L Signal at Trailing Edge Time>>
At a trailing edge time, in a case where the voltage at the input terminal In is lowered to be equal to or lower than the zener voltage of the zener diode ZD21, the potential at the positive input terminal of the operational amplifier AMP is the voltage at the input terminal In. On the other hand, the potential at the negative input terminal of the operational amplifier AMP is lower than the voltage at the input terminal In, because the voltage at the input terminal In is subjected to voltage division through the resistors R21 and R22. When the potential at the positive input terminal of the operational amplifier AMP is higher than that at the negative input terminal thereof, the output of the operational amplifier AMP has a logical HIGH level. Thus, the transistor Q21 is turned on. Electric current flows in the resistor R25, because the transistor Q21 is on. The input voltage detection portion ID outputs a signal, whose level is a logical LOW level, to the discharge control portion DC as an input detection signal. That is, at a trailing edge time, a signal whose level is a logical LOW level is output from the input voltage detection portion ID.
<Operation of Discharge Circuit Disclosed in Patent Document 2>
In a steady state in which the output voltage at the output terminal Out is maintained at a rated voltage, the input voltage to the input terminal In is substantially equal to the rated voltage. Thus, the input voltage detection portion ID outputs a signal whose level is a logical HIGH level (or off-signal (hereinafter represented by a logical level)) as an input detection signal to a terminal of the resistor R19, which is the input terminal of the discharge control portion DC. Because the transistor Q12 is turned off by inputting a logical HIGH level signal to the discharge control portion DC, an output of the discharge control portion DC is a logical LOW level signal (signal having a level equal to ground potential, or no signal). Accordingly, the input terminal of the switching element Q1 is at a ground level. Thus, the switching element Q1 is turned off. No discharge current flows in the discharge resistor R1. That is, in a steady state, the discharge current control portion DL feeds no discharge current.
That is, the discharge circuit according to Conventional Example 2 in FIG. 12 and FIG. 13 performs the on/off control of the switching element Q1 of the discharge current control portion DL and the discharge control via the discharge resistor R1, based on a control portion power supply voltage and an input detection signal, by detecting an input voltage with an input voltage detection portion ID connected to an input terminal, and by outputting to the discharge control portion DC an input detection signal according to the input voltage.
Referring next to FIG. 14, there is shown a discharge circuit according to Conventional Example 3.
<<Discharge Circuit according to Conventional Example 3>>
As illustrated in FIG. 14, the discharge circuit is configured so that when a discharge command signal FD1 is input to a photocoupler 36 of an insulation circuit, a gate drive PNP transistor 42 is turned on, that a gate drive voltage Vc1 is applied to a gate of a power metal-oxide semiconductor field-effect transistor (MOSFET) 40 via resistors 43 and 41, that thus, the power MOSFET 40 is turned on, and that a discharge operation is performed via a discharge resistor 39. The gate power supply circuit portion 30 has a resistor 31 connected to the positive terminal P of a high-voltage capacitor, a zener diode 33 which generates the gate drive voltage Vc1 and is connected to the negative terminal N of the high-voltage capacitor, and an electrolytic capacitor 34 which stores electric charges corresponding to a voltage developed across the zener diode 33 and supplies a gate drive power supply voltage.
<<Discharge Circuit according to Conventional Example 4>>
FIG. 15 illustrates a discharge circuit according to Conventional Example 4 in a case where an overheat protection circuit for a discharge circuit is added to the discharge circuit illustrated in FIG. 14.
As illustrated in FIG. 15, the discharge circuit is featured in that a forced discharge circuit portion 22a is provided with an overheat protection portion 28 for protecting a discharge resistor 39 from being overheated, so that the overheat protection portion 28 is disposed between the connection point of a gate resistor 41 of a power MOSFET 40 and the collector of a gate drive PNP transistor 42 and the negative terminal N of a high-voltage capacitor. A discharge current is detected by the voltage drop across a detection resistor 51. When an NPN transistor 47 is turned on by applying a voltage to the base of the NPN transistor 47, the base of a PNP transistor 46 is connected to a voltage of 0V of the gate power supply circuit portion 30. At that time, the emitter-collector junction of the PNP transistor 46 conducts to apply a base drive voltage to the base of the NPN transistor 47. Thus, the NPN transistor 47 maintains an on-state. In addition, the gate voltage of the power MOSFET 40 is dropped to 0V. Consequently, the power MOSFET 40 is turned off, and the discharge current is interrupted.
An operation of turning off the gate voltage is continued in the aforementioned manner. Accordingly, the discharge resistor 39 is protected from being overheated by performing a discharge operation in a high-voltage applied state.    [Patent Document 1] JP-A-6-276610 (page 4, and FIG. 1)    [Patent Document 2] JP-A-2003-235241 (Pages 6 to 7, and FIGS. 2 and 4)<Drawbacks of Conventional Discharge Circuit>
However, the high-voltage discharge circuit of the aforementioned conventional electric vehicle inverter apparatus is configured to constantly perform discharge. Accordingly, the discharge resistor is large in size. The deterioration of the mountability of the inverter apparatus to a vehicle and the increase in the manufacturing cost thereof occur due to the increase in the required space and the weight thereof.
In a case where the method of cooling the inverter apparatus is implemented by the air-cooling of the discharge resistor, when an electric vehicle runs at low speed, the cooling capability is reduced. Thus, at worst, the discharge resistor may be put into a burnout state. Consequently, it is indispensably necessary to install the inverter apparatus at a water-cooling portion of the vehicle. However, the conventional discharge circuit has a problem that accordingly, the mountability of the inverter apparatus to a vehicle is more deteriorated.
<<Drawbacks of Discharge Circuit according to Patent Document 1>>
In the discharge circuit of the configuration according to Conventional Example 1 illustrated in FIG. 11, which is disclosed in Patent Document 1, the normally closed type relay Ry3 is turned on after the power supply relay Ry1 and the charge relay Ry2 are opened. Thus, the high voltage of the smoothing capacitor C is discharged with the discharge resistor R1. At that time, in a case where an on-failure occurs in an excitation operation transistor of the power supply relay Ry1 or the charge relay Ry2, the excitation state of the power supply relay Ry1 or Ry2 is maintained. Thus, a high-voltage power supply Vdc remains connected to the discharge resistor R1. Consequently, the discharge circuit according to Patent Document 1 has a problem that the overheat destruction of the discharge resistor R1 is caused. The discharge circuit according to Patent Document 1 has another problem that in a case where the power supply relay Ry1 or the charge relay Ry2 is normally opened, even when the DC brushless motor M performs a regeneration operation or continues rotation, electric current continues to constantly flow in the discharge resistor R1, so that the discharge resistor R1 overheats.
<<Drawbacks of Discharge Circuit according to Patent Document 2>>
In the discharge circuit of the configuration according to Conventional Example 2 illustrated in FIGS. 12 and 13, which is disclosed in Patent Document 2, the input voltage and the output voltage are uninsulated from each other. A zero-volt line is shared by the input-terminal side and the output-terminal side. Thus, the discharge control is performed at the output side using input voltage detection information. On the other hand, in the inverter apparatus having the high voltage portion, the circuit control portion and the detection control portion are operated by the power supply circuit using a low-voltage battery, and thus cannot be configured to have the same potential as that of the high voltage portion. Consequently, the inverter apparatus provided with the high voltage portion using the discharge circuit according to Patent Document 2 has a problem that an insulation circuit is indispensably required.
<<Drawbacks of Discharge Circuit according to Conventional Example 3>>
The discharge circuit of the configuration according to Conventional Example 3 illustrated in FIG. 14 has the following problems. That is, there is a fear of occurrence of a state, in which a discharge operation is not completed, according to the signal state of the discharge command signal FD1, due to, e.g., chattering or the interruption of the low-voltage battery.
In addition, when a discharge signal is input in a high-voltage applied state, electric current constantly flows in the discharge resistor, so that the discharge resistor is overheated.
Additionally, in the gate power supply circuit portion 30 of the forced discharge circuit portion 22 in FIG. 14, the resistance value of the resistor 31 is set at high value, because the gate power supply voltage is low. Thus, when the inverter apparatus starts up, it takes long time until the apparatus reaches a gate power supply voltage at which a discharge operation can be performed.
<<Drawbacks of Discharge Circuit according to Conventional Example 4>>
In the discharge circuit of the configuration according to Conventional Example 4 illustrated in FIG. 15, the overheat protection circuit portion 28 is added to the circuit illustrated in FIG. 14 in order to prevent occurrence of the problem of overheat of the discharge resistor 39 illustrated in FIG. 14. Although the gate drive potential can be set at 0V and the discharge operation can be stopped when the overheat protection circuit portion 28 is disposed at such an installation position, the following defects may occur.
That is, because the power supply impedance of the gate power supply circuit portion 30 is high, the voltage of the storage electrolytic capacitor 34 is instantly dropped to 0V. During that, the gate drive PNP transistor 42 may be turned off. Thus, an amount of electric current flowing through the emitter-collector junction of the PNP transistor 46 is 0. The base current of the NPN transistor 47 is 0, so that the NPN transistor 47 is turned off, and that a latched state is canceled. In a case where the terminal voltage Vpn of a main circuit capacitor 7 remains in a high voltage state, and where a discharge command signal FD1 is input, when the storage electrolytic capacitor 34 is charged by the diode 38 via the resistor 31 to a level close to a zener voltage of the zener diode 33 for generating a gate power supply voltage, the PNP transistor 42 is turned on again. Thus, the power MOSFET 40 is turned on. Consequently, a discharge operation is started again. Discharge current flows therefrom via the discharge resistor 39. However, while the high-voltage applied state is continued, the overheat state occurs again. The discharge operation and an operation of stopping the discharge for overheat protection are intermittently repeated. Accordingly, there is a fear that the discharge resistor 39 may finally reach an overheat condition and a burnout state.